The present invention relates to a front gear for bicycles, and more particularly to a front gear or sprocket wheel for driving a chain for driving the rear wheel of a bicycle.
Conventional front gears for bicycles comprise a support member including a boss formed with a crankshaft bore and support arms extending radially from the boss, and an annular member formed with sprockets along its outer periphery. The support member and the annular member are made of metal. For assembling, the annular member is fastened to the support arms of the support member by bolts and nuts or by rivets, so that the procedure is cumbersome and costly. Further since the gear is made of metal in its entirety, it is difficult to color the gear variously for decoration.
It appears that the front gear can be produced at a lower cost and can be colored as desired if it is molded entirely from plastics, but in this case, the boss to which a crankshaft is to be attached is low in strength. Furthermore the support arms will not have sufficient strength to support the annular member having the sprockets. Thus the gear is unfit for use.
We thought it possible to overcome this problem by making the boss from metal and molding the annular sprocket portion and the support arms therefor integrally from plastics. In this case, however, the boss will not be joined to the support arms firmly but is likely to separate therefrom. Further when the metal boss is inserted in an arm mold and the support arms are formed from plastics as connected to the arms by injection molding, the sprocket portion formed will not be concentric with the boss if the boss is displaced from the specified position within the mold even slightly.